Computer Networks Workbook

An internet of internets — autonomous systems, end hosts, routers, the IP datagram contract that holds it together.

ARPANET → NSFNET → commercial Internet. The forty-year arc and the design decisions that ossified into today's stack.

Who actually governs the Internet? The bodies, the RFC process, the registries — how a protocol becomes interoperable.

Seven layers, what each one is for, why nobody actually deploys all seven, and where the model still helps reasoning.

Four layers (link, internet, transport, application) and the reason the Internet runs on them instead of OSI.

How a packet picks up headers on the way down and sheds them on the way up — the mental model for every layered protocol.

How end hosts get on the Internet — the last-mile technologies, their bandwidth shapes, and where they're going.

Guided vs unguided media, LAN/MAN/WAN/PAN, and the standard topologies (bus, ring, star, mesh) you'll still meet in datacenters.

What apps actually do on top of the transport layer — process-to-process messaging, architectures, and the protocols you'll meet.

Two app-layer architectures, their scaling shapes, and the hybrid (BitTorrent + trackers) that combines them.

The protocol most engineers know best. Methods, status families, the headers that actually matter, persistent connections.

Stateful sessions over a stateless protocol, Set-Cookie / Cookie headers, SameSite, Secure, HttpOnly, and the alternatives.

Root → TLD → authoritative → recursive; A / AAAA / CNAME / MX / NS / TXT; the recursive resolution walk.

Three protocols for one job. Why SMTP is push, POP/IMAP are pull, and where modern alternatives took over.

Tracker + swarm + pieces + tit-for-tat. The protocol that made P2P scale and still teaches the patterns.

End-to-end delivery over an unreliable IP network. Multiplexing, demultiplexing, and the reliability spectrum.

Connectionless, unreliable, lightweight. The header, the checksum, and the cases where UDP is the right answer.

Three-way handshake, four-way close, sequence/ack numbers, the flags (SYN/ACK/FIN/RST/PSH/URG), what each enables.

Stop-and-wait → Go-Back-N → Selective Repeat. The recipe TCP refines into its flow- and congestion-control machinery.

The receive window, advertised window, zero-window probes, and the window-scaling option that fixed the 64KB limit.

The four canonical algorithms (slow start, congestion avoidance, fast retransmit, fast recovery) and why AIMD converges to fairness.

Half-open connections, SYN cookies, retransmission timeouts — the security and resilience knobs on TCP.

Forwarding vs routing, the control plane vs the data plane — what turns 'a wire between two hosts' into 'the Internet'.

32-bit addresses, classless inter-domain routing (CIDR), subnet masks, fragmentation, the IPv4 header.

128-bit addresses, simplified header, no fragmentation by routers, IPv4 exhaustion as the forcing function.

ping, traceroute, destination-unreachable, time-exceeded, redirect — how the network tells you what went wrong.

Mapping IPv4 addresses to MAC addresses inside a subnet. The cache, the request/reply, and the spoofing risk.

DISCOVER / OFFER / REQUEST / ACK. The four-way handshake that gets your laptop an IP address.

Why everyone is behind a NAT, how it breaks the end-to-end principle, and the firewall stances (stateful vs stateless) you'll meet.

Hand-edited tables vs protocols that converge. When each is correct, and the operational cost of choosing wrong.

Bellman-Ford on the control plane — each router advertises distances to its neighbours, count-to-infinity, split-horizon, poison-reverse, and why RIP capped at 15 hops.

Link-state routing inside an AS, Dijkstra's algorithm, hello protocol, area hierarchies — the routing protocol of enterprises and ISPs.

Path-vector routing between ASes, policy-based selection, the trust model that makes BGP both essential and fragile.

A single HTTP request from address-bar keypress to rendered page — ARP, DHCP, DNS, IP, TCP, HTTP and back, narrated layer by layer.

Framing, error detection, and medium access — what the link layer adds on top of raw signal.

Pre-allocated time, frequency, or code slots. The schemes mobile and satellite links rely on, and why static beats stochastic for them.

The original stochastic medium-access protocol from 1971 Hawaii, the throughput math (18% and 37%), and what slotting bought.

Stochastic medium access: listen, defer, transmit, detect collisions. The mechanism behind classic Ethernet and Wi-Fi.

Preamble + dst + src + EtherType + payload + FCS; the switch fabric; VLAN tagging (802.1Q); the spanning tree.

How a switched LAN avoids loops — BPDUs, root-bridge election, port roles, blocked links — and why RSTP and MSTP exist.

PHY layers (a/b/g/n/ac/ax/be), CSMA/CA, association, hidden terminals, RTS/CTS, the security generations (WEP→WPA→WPA3).

The six commands you actually use to debug networks. Read packets, follow paths, resolve names, talk HTTP.

socket() + bind() + sendto/recvfrom. The simplest network programs that teach the model.

A small but realistic project: many clients, one relay server, broadcast over UDP. The framing, the keep-alive, the cleanup.

socket() + connect/listen/accept + sendall/recv. Custom framing, blocking vs non-blocking, the patterns that scale.

Build a small network of router processes that exchange distance vectors, converge to shortest paths, and survive a link failure.

From a link-state database to a forwarding table in O((V+E) log V). The algorithm OSPF runs on every router.

A loop-free spanning tree over a switched-LAN graph: root election, port roles, BPDU exchange, the convergence walk.

A bad WAF regex with exponential backtracking; 30 minutes of global edge downtime; the postmortem on safe deploys.

A maintenance command withdrew BGP routes for Facebook's nameservers; 6+ hours of global outage; the lessons on out-of-band access.

A misconfigured command in the S3 billing system; cascading failure through us-east-1; the lessons on blast radius.